Characteristics of the depositional systems and sequence evolution of the Yanchang Formation in the southern Ordos basin

The depositional systems of the Yanchang Formation in the south of the Ordos basin originated in different geological settings and have their different provenance directions and depositional environments. Thus they show appreciable differences in depositional and distribution characteristics. Take for example oil set Chang 8 of the main oil section in the Ordos basin. The braided-stream delta depositional systems are located in a steep slope zone on the southwestern margin of the Ordos basin; their sand bodies consist of arkose and are characterized by development of trough bedding and occurrence of gravity-flow deposits in the prodelta. Gravity-flow deposits, mainly distributed in the Chang 7 oil section in the Weibei area, southern Ordos basin, consist predominantly of lithic sandstone and formed in a deep lake environment, where the Bouma sequence includes the ABCE, ADE, AE and BCE divisions. The meandering-river delta system formed by the northeast-directed river system lies mainly in a gentle slope zone in the northeast of the basin; the distributary channels of the delta extend over a long distance and their sand bodies are large in scale, most of which consist predominantly of feldspathic fine sandstone, and debouch bars are well developed.